The present invention comprises an apparatus and process which are used to rapidly cool hot fluids. The apparatus comprises a means whereby a hot fluid is contacted on cooling surfaces to provide a rapid decrease in the temperature of the hot fluid. The cooling means is suitable for use with cooling fluids at elevated pressures. Further, the apparatus and process of the present invention rapidly cools hot gases without substantial pressure change in the process stream.
The cooling means has particular and advantageous application when used in conjunction with a process for the production of olefins by cracking hydrocarbon feeds, such as light naphthas at high temperature and short residence time, using a high radiant heat furnace having relatively short conduits of small diameter. The cooling means provides a rapid reduction in the effluent product gas temperature from the furnace without also incurring a substantial pressure drop.
Cracking furnace effluent gas temperatures are very high and at these high temperatures the cracking reactions proceed at a rapid rate. In order to substantially stop the reactions in the effluent gas to reduce the production of undesirable by-products, especially coke precursors such as pyrolysis fuel oil, it is beneficial to rapidly cool the effluent gas after it leaves the reactor to a temperature at which the reactions substantially cease. There are several conventional relatively high capacity means available for doing this, most of which have one or more drawbacks. Most commonly, the conventional means of cooling comprises shell and tube heat exchangers employing tube sheets which invariably result in substantial pressure loss of the effluent gas. This type of heat exchanger employs a tube sheet having multiple tubes at the inlet head which causes the formation of eddies and back-mixing. See FIG. 3.
The eddies are detrimental to the production of olefins for two main reasons. Firstly, the eddies increase the pressure lost at the inlet head, thereby raising the pressure at the reactor coil outlet and thus reducing the yield of olefins. Secondly, the eddies constitute volumes of backmixing with higher than average residence times. At the temperatures present in the inlet head to the cooling zone of the quench boiler, even small increases in residence time, i.e., as low as about 5 milliseconds, result in significant overcracking of the feed to tar and coke, and concomitantly in reactor shutdown for decoking operations.
Mention is also made herein of Woebcke, U.S. Pat. No. 3,910,347, which teaches a relatively low capacity quench boiler essentially tubular in shape and provided with a divergent inlet section having an angle of divergence of less than 10.degree.. While this device has met with considerable success in the industry to date, increasing demands of product selectivity, lower quality feedstocks and increased capacity requirements, have resulted in the need for improved relatively high capacity quench boilers which are also effective in heat recovery.
The cooling techniques and apparati of the present invention are particularly useful in the high capacity cooling of the effluent gases from a process for the thermal cracking of hydrocarbons. In the thermal cracking of hydrocarbons, in the process described hereinafter, the hydrocarbon feed can be heated to high temperature, maintained at high temperature for a short residence time and selectively converted to desired products. In accordance with the present invention, the hot gas reaction products are rapidly quenched or cooled in such a manner that the conversion is substantially stopped after the desired residence time. Still further, the apparatus of the present invention can be employed in further reducing the temperature of the reactor effluent to within about 100.degree. F. of the coolant medium, thereby providing an excellent means of heat recovery and steam generation.
Although the quenching apparatus and process of cooling of the present invention are particularly suitable for use in cooling the hot gas effluent issuing from a pyrolysis furnace, the concept can readily be applied to other processes for cooling hot product streams, for heat recovery and/or for heating fluids. The cooling means and process can be used for rapidly cooling hot gaseous products from other cracking processes. The quenching apparatus provides indirect cooling on surfaces. The apparatus is simple in design and easy to operate. The apparatus can be of any size and is normally designed for a specific processing facility. The apparatus can be vertical or horizontal. The cooling unit rapidly cools hot fluids while not substantially changing the pressure of the fluid. That is, the pressure of the cooled fluid at the outlet of the cooling is substantially the same as the inlet pressure. The material to be cooled can be upflow or downflow.